Strong variation in frequencies of male and female determiners between neighboring housefly populations.

Sex-determination mechanisms evolve rapidly and vary between species. Occasionally, polymorphic systems are found, like in the housefly. Studying the dynamics and stability of such systems can provide a better understanding of the evolution of sex-determination systems. In the housefly, dominant male-determining loci (M) can lie not only on the Y chromosome (M Y ), but also on autosomes (M A ) or the X chromosome (M X ). M enforces male development by inhibiting the female-determining gene transformer (tra). A mutant tra allele, tra D , is insensitive to M and is a dominant female determiner. M Y prevails at high latitudes and polymorphic M loci together with tra D at low latitudes. To get more insight into the stability and frequencies of these sex determiners with mutually exclusive dominance, we investigated 5 regional Spanish populations. We found strong variation among populations. Two populations with hemizygous M III were found, 2 contained homozygous M X with additional hemizygous M I and M II in 1 population. One population contained homozygous and hemizygous M X with additionally hemizygous M II . All females in populations with homozygous M, had tra D , whereas no tra D was found in populations without homozygous M. Our results indicate locally stable systems may either harbor a single hemizygous M and no tra D , corresponding to a male heterogametic system, or homozygous and/or multiple M and heterozygous tra D , reminiscent of a female heterogametic system. They support that M loci can accumulate in the presence of a dominant female determiner. Limited migration between populations may contribute to the stability of these systems.